The invention relates to an electromagnetic adjustment unit of a hydraulic directional control valve with a housing, at least one armature and at least one magnet yoke. The magnet yoke has a tubular section adjacent to an annular section having a greater outer diameter in the axial direction. Within the magnet yoke there is a tappet push rod, which can move axially and which can be displaced by means of the armature. The housing has a pot-like shape with a base, in which there is an opening, through which the tubular section extends into the housing, the outer diameter of the tubular section is at least approximately adapted to the diameter of the opening, and the base is supported indirectly or directly on the annular section.
Such directional control valves are used in internal combustion engines, for example, for controlling hydraulic camshaft adjusters or switchable cam followers. The directional control valves comprise an electromagnetic adjustment unit and a valve section. The valve section represents the hydraulic section of the directional control valve, wherein at least one supply connection, at least one working connection, and one tank connection are formed on the hydraulic section. By means of the electromagnetic adjustment unit, certain connections of the valve section can be selectively connected to each other hydraulically and in this way the flows of pressurized medium are guided.
For the use of a directional control valve to control a camshaft adjuster, in the normal case this valve is formed as a 4/3 proportional directional control valve. Such a proportional valve is disclosed, for example, in DE 199 56 160. In this case, the electromagnetic adjustment unit is composed of a first magnet yoke, a coil, a second magnet yoke, a housing, an armature, and a connection element, which receives an electrical plug connection used for supplying power to the coil.
The valve section is composed of a valve housing and a control piston that can be displaced axially therein. The valve housing is arranged within a receptacle opening of the second magnet yoke and is connected to this yoke so that it is fixed in position. On the outer jacket surface of the valve housing, there are four annular grooves, which are used as pressurized medium connections. Openings are formed in the groove bottoms of the annular grooves, whereby pressurized medium can be led into the interior of the valve housing. In the interior of the valve housing, there is a control piston that can be displaced axially, wherein the outer diameter of the control piston is adapted to the inner diameter of the valve housing. In addition, annular grooves are also formed on the control piston. Adjacent pressurized medium connections can be connected to each other via these annular grooves.
The coil and the first and second magnet yokes are arranged coaxial relative to each other within the housing of the electromagnetic adjustment unit. The first and the second magnet yoke are offset relative to each other in the axial direction. In the region between the first and the second magnet yoke, there is the armature radially within the magnet yokes, wherein this armature is surrounded by the coil in the radial direction. The armature, the housing, the first and the second magnet yokes form a flow path for the magnetic flux lines, which are generated by sending current through the coil.
By sending current through the coil, the armature is forced in the direction of the second magnet yoke, wherein this motion is transferred to the control piston by means of a tappet push rod attached to the armature. This piston is now moved in the axial direction against a spring supported against the valve housing.
For the most part, directional control valves for controlling switchable cam followers are embodied as switching valves. Such a switching valve is known in a configuration as a 3/2 switching valve, for example, from DE 102 52 431 A1. The electromagnetic adjustment unit comprises, in turn, a housing, an armature, a connection element, and a first and a second magnet yoke. The function and the configuration of the electromagnetic adjustment unit are in broad parts analogous to that of the proportional valve.
In this case, a supply connection, a working connection, and a tank connection are formed on the valve section. The working connection communicates via openings, which are each formed as valve seats, both with the supply connection and also with the tank connection. Furthermore, within the valve housing there is a control piston, on which two closing elements are formed. Each closing element can block or open the pressurized medium flow through one of the valve seats depending on the position of the control piston within the valve housing. Depending on the axial position of the control piston, the working connection can be connected selectively to the supply connection or to the tank connection. The axial position of the control piston is fixed relative to the second magnet yoke, in turn, over the axial position of the armature.
The assembly of the electromagnetic adjustment unit produces relatively high assembly expenditure, whereby higher costs arise, due to the plurality of components, which must be positioned and fixed relative to each other.